This invention generally relates to an imaging reader for, and a method of, reading an optical code by visually positioning a laser-based aiming pattern relative to the code to be read during an aiming mode of operation and, more particularly, relates to enhancing detection of the aiming pattern during the aiming mode without violating human eye exposure laser safety limit standards and without reducing visibility of the aiming pattern to a user, especially in the presence of bright ambient light.
Imaging readers have been developed heretofore for reading optical codes, such as one- or two-dimensional bar code symbols, appearing on a label or on a surface of an article. The symbol itself is a coded pattern of graphic indicia comprised of, for example, a series of bars of various widths spaced apart from one another to bound spaces of various widths, where the bars and spaces have different light reflecting characteristics. The imaging readers electro-optically transform the graphic indicia into electrical signals, which are decoded into alphanumeric characters that are intended to be descriptive of the article or some characteristic thereof. Such characters are typically represented in digital form and utilized as an input to a data processing system for applications in point-of-sale processing, inventory control and the like.
Imaging readers are used in both fixed and portable installations in many diverse environments, such as in stores for check-out services, in manufacturing locations for work flow and inventory control, and in transport vehicles for tracking package handling. The readers can be used for rapid data entry or for self-configuration, such as by scanning a target bar code symbol from a printed listing of many bar code symbols. In some uses, the reader is connected to a portable data processing terminal or a data collection and transmission terminal. Frequently, the reader is handheld. Often, the reader is a single component of a much larger system or network including other readers, computers, cabling, data terminals, etc.
The imaging reader has an imager module, which includes an image sensor having a one- or two-dimensional, solid-state array of cells or photosensors, such as a charge coupled device (CCD), a complementary metal oxide semiconductor (CMOS) device, a charge modulated device (CMD), or a charge injection device (CID). The imager module images a target symbol by sensing return light reflected or scattered off the target symbol being imaged over an exposure time period, and by responsively generating a plurality of electrical signals corresponding to a one- or two-dimensional array of pixel information describing a field of view (FOV) of the imager module. The electrical signals are then processed and provided to decode circuitry for decoding thereof. The imager module typically includes a lens assembly for capturing and focusing the return light on the image sensor.
When multiple optical codes are in the FOV of the imager module, for example, in the case of a shipping label on which multiple bar code symbols are printed, a known reader of the prior art typically automatically determines which code is the easiest to capture and/or read, and that code is decoded first. A user of this known reader does not control which code the reader should try to decode and, accordingly, may have difficulty scanning a desired code.
To assist the user in scanning the desired code, imaging readers are often equipped with an aiming assembly having an aiming laser for generating a laser beam, and optics for generating a visible aiming pattern, such as a “crosshair” pattern, from the laser beam. The user trains the aiming pattern on the target symbol to be imaged during an aiming mode. In commercially available imaging readers, it is common for a center of the aiming pattern to not coincide with a center of the FOV of the imager module due to parallax and to mechanical or manufacturing inconsistencies, including the displacement between the laser of the aiming assembly and a focal point of the lens assembly for focusing light onto the image sensor. The user may use the aiming pattern to choose a desired code that is presented together with multiple optical codes, such as on a sheet or label having one or more columns of printed optical codes. The user may try to align the center of the aiming pattern to coincide with, or be nearest to, the desired code and then manually activate a reading mode, such as by pulling a trigger.
Upon activation of the reading mode, the reader temporarily disables generation of the aiming pattern so that the aiming pattern is not incorporated into the image being acquired in order not to obstruct the target code being imaged. The actual position of the aiming pattern in the acquired image is not necessarily in the center of the acquired image. In fact, the actual position of the aiming pattern is not known. The desired code is not necessarily the acquired code that is closest to the center of the acquired image. U.S. Patent Application Publication No. 2006/0043191 discloses a reliable way for the image sensor to determine which optical code of the multiple optical codes lying within the FOV of the imager module is the desired optical code. Thus, the image sensor captures an image of the aiming pattern and of the target code during the aiming mode in order to determine the desired optical code, and captures an image of just the desired optical code during the reading mode.
Nevertheless, as advantageous as these known imaging readers have been in targeting the desired optical code, there are situations when the aiming patterns cannot be readily detected by the image sensor during the aiming mode. In some applications, such as in well-lit indoor environments or outdoors in sunlight, the aiming patterns can be washed out by the brighter ambient light. If the image sensor cannot detect the aiming pattern during the aiming mode, then the desired optical code may not be detected and read.
Detectability of the aiming pattern in the image captured by the image sensor is proportional to the contrast of the aiming pattern compared to the rest of the captured image. Increasing the intensity of the aiming laser beam will increase the brightness or detectability of the aiming pattern. However, increasing the beam intensity may violate human eye exposure laser safety standard limits. For example, a class 2 laser is limited to an output power of 1 mW over a base time interval of 250 msec, and a class 1 laser is limited to an output power of 0.39 mW over a base time interval of 10 sec. The beam intensity cannot exceed these limits. Conversely, decreasing the beam intensity may result in the aiming pattern being undetectable by the image sensor and not visible to the user.
Accordingly, there is a need for a system for, and a method of, enhancing the detection of the aiming pattern in the image acquired by the sensor during the aiming mode, especially when a desired optical code is situated among multiple optical codes in the FOV of the imager module, particularly under bright ambient light conditions, without violating human eye exposure laser safety limit standards and without reducing visibility of the aiming pattern to a user.